System for repulsion development in electrophotography

ABSTRACT

System for developing electrostatic images by repulsion development to allow full development of areas bearing uniform charge. An insulating developing electrode bearing charge controlled electrostatically to decay at the same rate as charge from the latent image is placed adjacent to the latent image bearing surface, and both are spaced a uniform distance from a conductor. This allows the electric field produced to become zero in that part to which no particles are to adhere.

Q Umted States Patent 1 1 3,622,314 72] Inventors Ylsuo Tamil; [56] References Cited Sim Mllslll lom; Mlsllllkhl S010; Satoru UN|TED STATES PATENTS 2] A I N i;"' 2,573,881 11/1951 Walkup et :11 117 175 x R, Jul 1 1968 2,777,418 1/1957 Gundlach..... 117/17.sx gf 5 19" 2,817,598 12/1957 Hayford 117/175 2,877,132 3/1959 Matthews.. 117/17.5 [73] fl 3" 3,147,147 9/1964 Carlson 118/637 [32] Priority 531'? 3,311,490 3/1967 Fawseretal 117/37 1.x [33] Japan Primary Examiner-Charles E. Van Horn 31 42/4253 Attorney-Sughruc, Rothwell, Mion, Zinn & Macpeak [54] SYSTEM FOR REPULSION DEVELOPMENT [N ABSTRACT: System for developing electrostatic images by ELECTROPHOTOGRAPHY repulsion development to allow full development of areas ICInIm, 1 Drawing Fig, bearing uniform charge. An insulating developing electrode 52 U S Cl 96 1 bearing charge controlled electrostatically to decay at the f l same rate as charge from the latent image is placed adjacent to 5 I Cl G03l3 the latent image bearing surface, and both are spaced a d 968/] uniform distance from a conductor. This allows the electric ll7/l7.5, 37 LE; 118/637 field produced to become zero in that part to which no particles are to adhere.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention concerns the development of electrostatic images, and more particularly concerns the so-called "repulsion development process under which more developing (i.e. toner) particles are caused to stick to less charged portions of the charged pattern (electrostatic latent image) formed on an insulating surface and no developing particles are caused to stick to portions having the maximum charge density.

2. Description of the Prior Art As is well-known, if developing particles having the same polarity as the charge forming an electrostatic latent image on an insulating surface (for instance, a photoconductive insulating layer) are caused to draw near the insulating surface, such particles are repelled by electrostatic repulsion force on heavily charged surface areas and they stick to lightly charged or uncharged surface areas. However, in this process, developing particles do not stick to an image because of an electrostatic attractive force. That is, they stick according to the intensity of the surface electric field, not according to the density of the charge fonning the electrostatic latent image. Accordingly the particles adhere in large quantities to portions where the difference in charge density is large, whereas they scarcely stick to a portion uniformly bearing less charge. To eliminate this disadvantage, a method is known, in which a DC voltage, equal to the surface potential of the portion of the electrostatic latent image where charge density is the highest, is applied to a developing electrode. An electric field proportional to the difference between an image portion bearing less charge and an image portion bearing charge at the highest density is produced between the developing electrode and an insulating surface to carry out satisfactory development.

However, the surface potential in the portion of an insulating surface bearing electrostatic latent images gradually decays with time in darkness. The characteristic of such darkness decay varies greatly depending on the type of insulating surface and environmental conditions. The decay characteristics are not simple enough to be determined by a given time constant. Consequently, it is extremely difficult to change the voltage to be applied to a developing electrode according to such darkness decay.

SUMMARY OF THE INVENTION The present invention provides a novel method which eliminates this disadvantage and in which an electrostatic latent image on an insulating surface is developed while compensatively adjusting the potential of an electric field control element, equivalent to a developing electrode, generally according to darkness decay. Using this method, it is not necessary to impress voltage from an external power source upon a developing electrode during the developing process. The method allows an image of continuous gradation, having no edge effect, to be obtained.

That is, the present invention relates to a method which, in the so-called repulsion development process," in which, by employing developer containing developing particles charged in the same polarity as the electrostatic latent image formed on an insulating surface, the developing particles are caused to stick more to an area of the electrostatic latent image where charge density is low. The disclosed method develops the electrostatic latent image under the following condition: The electrostatic latent image portion, having area S and having potential V, at the maximum charge density portion, and a uniformly charged insulating surface, having the same characteristics as the insulating surface bearing the electrostatic latent image, and having area S and potential V,, are placed to face a conductor with a uniform small gap and the following requirement is satisfied:

VITSHYSBYZ The above mentioned same characteristics" means that the uniformly charged insulating surface and the insulating surface bearing the electrostatic latent image have the same decay characteristic. It is preferable in the present condition that photoconductive layers of the same composition are used. ,for both of these insulating surfaces. In this case, all the influences due to the change of atmosphere (moisture, temperature, etc.) are all automatically compensated. Of course, different types of insulating surfaces with the same decay characti asses BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a cross-sectional view of an arrangement of insulating material, conductor, and support material used in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention will now be explained with reference to the single FIGURE. The FIGURE illustrates an example of application of an insulating material formed by providing a supporting material 1 with a sensitivelayer 2 of photoconducitive insulating material. Part A, called the first region of this sensitive layer 2, is charged uniformly, for example, to nega- ;tive polarity. Thereafter the first region is image-exposed to .form an electrostatic latent image, while a remaining part B, called the second region of sensitive layer 2, is uniformly charged to the same polarity. The surface of sensitive layer 2 is placed quite near and opposite to a conductor 3, leaving a uniform space between them. Because development is carried out by negatively charged developing particles, the direction of electric line of force is expressed oppositely to the ordinary expression corresponding to the direction of the force to which said developing particles are subjected. The electric lines of force proceeding from second region B toward the adjacent conductor 3 are distributed in a generally uniform density. In other words, positive charges are induced in conductor 3. In the first region A, electric lines of force proceed from the conductor 3 toward the electrostatic latent image-bearing surface. The density of these electric lines of force depends on the area ratio of the first region A and the second region B, and the potential in second region B. The area of first region A is assumed to be S,. The voltage of the part corresponding to the maximum charge density portion, that is, the part 4 to which no developing particles are to stick is assumed to be V The area of the second region B is assumed to be 8 and its surface voltage is assumed to be V Thus in the case where S, and V are respectively larger than S and V,, the density of electric lines of force proceeding toward the electrostatic latent image-bearing surface is higher in the first region A. Accordingly negative developing particles can easily stick to the surface of the electrostatic latent image. In this case, corresponding to the distribution of charges in first region A, electric lines of force emanate in a reverse direction and gradation is thus rendered. But fogging will be produced unless 8,, 8,, V, and V are so selected that the electric field ,becomes zero in that portion in the first region A to which no ;developing particles are to adhere. After consideration of these theoretical conditions, and as a result of experiments, it has been found that satisfactory development, without fogging, can be achieved where the condition of is satisfied. In practice, there are cases where the occurrence of a little fogging is allowable and therefore, even under the condition of the disadvantage of tion is shown on right side of the fonnula.

Next, an example of practical application of the present invention is given.

Rectangular electrophotographic sensitive paper, in a sheet 200 a base sheet provided with a low-resistance primer coat. Half of the sheet, that is, a 100 mm. X 100 mm. part was shaded by means of a metal plate and the other half negative corona and charged to about 150 V. plate was removed and the entire surface of this electrophotographic sensitive paper was subjected to negative corona discharge which was stopped at the time when the newly charged portion (first region) reached 150 V and the twice charged portion (second region) reached 300 V. second region was covered with a sheet of tremely low in dark decay in air and nonpolar liquid. The sensitive paper was dipped, with its sensitive surface facing down, into a stainless steel vat having a flat bottom and filled with liquid developer comprising negatively charged black developing particles suspended in kerosene. After maintaining for 2 minutes the sensitive paper nearly in touch with the botof the original.

This practical example meets the requirements of the present invention. When, independently of this example,

tions of the present invention, whereas fogs are produced in the highlight portion upon deviation from these conditions.

As is clear from the above description, the energy conventionally impressed upon a developing electrode from an external power source may be considered to have been replaced by the energy of uniformly charged surface under the present invention.

The advantages of the method of the present invention are enumerated in the following.

1. An external power source for a developing electrode is not required. 2. The potential of electrostatic latent image varies with the lapse of time due to decay. Because of this, under the conventional methods employing a developing electrode,

present invention, charges in a uniformly charged region (the second region) decay in the same manner as in the image-bearing region, with the influence being thus compensated for automatically.

3. An image of continuous gradation not involving edge effect can be obtained under a simple process.

Under the method of the present invention, if the supporting medium of the insulating material such as paper does not possess satisfactory metallic conductivity, and if the first and second regions are cut off and each caused to face a common conductor separately, electrical connection of two supporting materials is insufficient, and the effect according to the bring about any marked difference in of the present application is only limited to the extent of the claims which follow.

We claim:

1. A process for developing electrostatic images by repulsion development comprising the steps of; providing a first inuniformly charging said first and second insulating surfaces to the same polarity; imageexposing the area S comprising said said conductor being unbiased; applying developer particles charged to the same polarity as said latent 

